Marijuana Doesn’t Shrink Brain Study Reveals

Marijuana doesn’t appear to shrink important regions of the brains of users, but a study published Wednesday shows something possibly more subtle and important: the brains of people who tend to use marijuana may be smaller to start with.

In the study, David Pagliaccio, formerly of Washington University in St. Louis and now at the National Institute of Mental Health, and colleagues used magnetic resonance imaging to compare the brains of marijuana users to non-users.

They included batches of siblings, one who used marijuana and the other who didn’t, to try to tease out whether brain differences might be genetic instead of being linked to marijuana use.

When they looked at all 482 volunteers, it did seem that the marijuana users had some shrinkage in two brain regions called the amygdala and the right ventral striatum. But when they compared marijuana users to their siblings, the differences disappeared. So it’s possible that people who choose to use marijuana may already have the smaller regions.

It’s not clear why, Goldman said. “It could be nutrition, it could be stress exposures, it could be a lot of different things,” he said.